Before the introduction of bulk curing in the early 1960's, tobacco was flue-cured in curing barns by means of heated air in a period of about six days. In conventional or flue-curing, the initial procedures involve stringing groups of tobacco leaves, also known as hands, to a tobacco stick. A string is used to tie the stems of the leaves comprising each hand together and to successively fasten a plurality of hands to a tobacco stick. The strung tobacco sticks are then supported in vertically and horizontally spaced relation on horizontal tier poles within the curing barn. To effect the curing, burners within the barn heat the air and establish a natural convection current from the bottom of the barn, through the leaves, and out the top of the barn.
The advent of bulk curing in the early 1960's reduced the amount of labor required in handling the tobacco leaves during the curing operation by eliminating the stringing procedures. Instead of stringing the tobacco on sticks, the leaves are loaded in mass within bulk curing racks which hold the tobacco leaf in a generally compacted bulk mass with the flat surfaces of the leaves extending generally in a vertical direction. Because of the compact relation of the leaves, bulk curing requires the forced air flow through the compact, bulk mass of leaves in order to affect curing of the leaves.
The curing of tobacco, whether in the conventional manner or by bulk curing, has essentially three stages:
1. The yellowing stage;
2. The leaf drying or color setting stage; and
3. The stem drying or killing out stage.
During the yellowing stage, the green color of the leaf produced by the cholorphyll content vanishes as the cholorphyll breaks down and a yellow color resulting from the carotene content appears. Another important change during the yellowing stage is the conversion of starch to sugar. Although independent, the conversion of starch to sugar and the breakdown of cholorphyll occur simultaneously. Thus, yellowing is a useful, visual measure of sugar formation. During the leaf drying and stem drying stages, the temperature of the air is gradually increased to affect the removal of moisture first from the mesophyll and then the stem of the tobacco leaves.
Impatience in curing frequently result in the temperature being raised too rapidly, causing inter alia, insufficient fermentation, brown scald, etc. For instance, if the temperature is advanced before the yellowing stage is completed, the green color may be set and certain chemical and biological processes which convert the starch to sugar will be stopped. Then the cured tobacco may give a harsh and irritating smoke. Similarly, if the temperature is advanced too rapidly and while there is excessive moisture in the leaf, the oxidation rate of polyphenols within the leaf will be excessive resulting in brown scald. Impatience in curing may also result in allowing insufficient time to "kill" the stems resulting in what is known as swell stem. These conditions all effect the quality and therefore the price of the cured tobacco.